easy check: take the cable loose from the speedo [just reach up behind the gauge set, a big finger tight knurl knob] - drive the truck and check the top end hanging at yer knee to see if it's turning - if it is, the speedo needs work, if not then it's likely a bad cable, next step is to pull the inner cable out of the housing from the top, if it's all there with the correct shaped ends, the drive gear in the trans is most likely the problem, if it's broke/gummy/twisted up, replacements are easily available
Bill
